1. Field of the Invention
This invention relates to an electrically conductive polymer composition exhibiting positive temperature coefficient (PTC) of electrical resistance behavior. Said composition can be used in PTC devices.
2. Introduction to the Invention
Conductive polymer compositions which exhibit PTC (positive temperature coefficient of resistance ) behavior are well-known for use in electrical devices such as circuit protection devices. Such compositions comprise a polymeric component, and dispersed therein, a particulate conductive filler such as carbon black or metal. The amount and type of filler in the composition are determined by the required resistivity for each application, as well as by the nature of the polymeric component. Compositions suitable for use in circuit protection devices have low resistivities at room temperature, e.g. less than 100 ohm-cm, and generally comprise relatively high levels of conductive filler.
Compositions with low resistivity are desirable for use in circuit protection devices which respond to changes in ambient temperature and/or current conditions. Under normal conditions, a circuit protection device remains in a low temperature, low resistance state in series with a load in an electrical circuit. When exposed to an overcurrent or overtemperature condition, however, the device increases in resistance, effectively shutting down the current flow to the load in the circuit. For many applications it is desirable that the device have as low a resistance as possible in order to minimize the effect on the resistance of the electrical circuit during normal operation. Although low resistance devices can be made by changing dimensions, e.g. making the distance between the electrodes very small or the device area very large, small devices are preferred because they occupy less space on a circuit board and generally have desirable thermal properties. The most common technique to achieve a small device is to use a composition that has a low resistivity.
The resistivity of a conductive polymer composition can be decreased by adding more conductive filler, but this process can affect the processability of the composition, e.g. by increasing the viscosity. Furthermore, the addition of conductive filler generally reduces the size of the PTC anomaly, i.e. the size of the increase in resistivity of the composition in response to an increase in temperature, generally over a relatively small temperature range. The required PTC anomaly is determined by the applied voltage and the application.
Japanese Patent Kokai Publication No. 172001/1996 (Heisei 08-172001) discloses that metal particles and metal-coated particles are used as the electrically conductive particles, because it is difficult to achieve electrically conductive material having a volume resistivity of at most 1 ohm-cm and good PTC anomaly when carbon black is used as the electrically conductive particles. However, the amount of the electrically conductive particles must be increased to decrease the resistivity. When the amount of the electrically conductive particles is increased, it is impossible to give sufficient PTC anomaly and molding of the composition is difficult due to poor flowability of the composition. Actually, the resultant value of the volume resistivity is limited.
Japanese Patent Kokai Publication No. 6309/1981 (Showa 56-6309) discloses a temperature sensor comprising electrically conductive particles dispersed in an insulative matrix. The insulative matrix comprises an aluminum soap added to a hydrocarbon wax. However, this temperature sensor does not exhibit sufficient PTC behavior.
Japanese Patent Kokai Publication No. 168005/1999 (Heisei 11-168005) discloses an organic PTC thermistor comprising an electrically conductive composition comprising a thermoplastic polymer matrix, a low molecular weight organic compound and electrically conductive particles. This publication describes that hydrocarbons, fatty acids, fatty acid esters, fatty acid amides, aliphatic amines and higher alcohols are used as the low molecular weight organic compound, but does not describe that a polymer is used as the low molecular weight organic compound. The electrically conductive composition has poor processability and does not have good PTC anomaly.
Hitherto, electrically conductive compositions having low volume resistivity have been obtained by adding a large amount of electrically conductive particles such as carbon black and metal powder to a matrix such as a polymer. However, electrically conductive compositions having satisfactory PTC anomaly cannot be obtained.
An object of the present invention is to provide an electrically conductive composition having good flowability at high temperature and low resistivity at 20xc2x0 C. and exhibiting good PTC anomaly.
In a first aspect, the present invention provides an electrically conductive polymer composition exhibiting positive temperature coefficient (PTC) of electrical resistance behavior and comprising:
(1) a polymer mixture comprising:
(i) at least 50% by volume of a first crystalline polymer having a weight-average molecular weight of at least 50,000, and
(ii) at most 50% by volume of a second crystalline polymer having a weight-average molecular weight of at most 10,000, and
(2) a particulate electrically conductive filler dispersed in the polymer mixture.
In a second aspect, the present invention provides a PTC device comprising:
(A) a PTC element (for example, a laminar PTC element) comprising the composition, and of the first aspect of the invention.
(B) two electrodes which can be connected to an electrical power source to pass an electrical current through the PTC element.
In a third aspect, the present invention provides an electrical circuit which comprises:
(I) the PTC device; of the second aspect of the invention.
(II) an electrical power source; and
(III) a load connected in series with the device and the power source.
The electrically conductive polymer composition of the present invention comprises a polymer mixture comprising a first crystalline polymer and a second crystalline polymer, and a particulate electrically conductive filler, and exhibits positive temperature coefficient (PTC) of electrical resistance behavior.
The polymer mixture comprises a first crystalline polymer and a second crystalline polymer. Preferably, the amount of the polymer mixture is from 20 to 90% by volume, more preferably 20 to 70% by volume, especially 30 to 70% by volume, based on total volume of the electrically conductive polymer composition.
The first crystalline polymer has a weight-average molecular weight of at least 50,000. The lower limit of the weight-average molecular weight of the first crystalline polymer is 50,000, preferably 100,000. The upper limit of the weight-average molecular weight of the first crystalline polymer is generally 10,000,000, e.g. 3,000,000, preferably 1,000,000, more preferably 600,000.
The crystallinity of the first crystalline polymer may be at least 10%, preferably at least 20%, more preferably at least 30%, especially at least 40%, e.g. from 50 to 98%.
The first crystalline polymer is generally a thermoplastic resin. Preferably, the first crystalline polymer is a polymer comprising at least one monomer selected from olefins or olefin derivatives, e.g. a homopolymer or copolymer of ethylene. Suitable examples of the first crystalline polymer include polymers of one or more olefins such as high density polyethylene; copolymers of at least one olefin and at least one monomer copolymerisable therewith such as ethylene/acrylic acid, ethylene/ethyl acrylate, ethylene/vinyl acetate, and ethylene/butyl acrylate copolymers; melt-shapeable fluoropolymers such as polyvinylidene fluoride and ethylene/tetrafluoroethylene copolymers; and blends of two or more such polymers.
The amount of the first crystalline polymer is at least 50% by volume, e.g. at least 60% by volume, particularly at least 70% by volume, especially at least 80% by volume, based on the polymer mixture.
The second crystalline polymer has a weight-average molecular weight of at most 10,000. Preferably, the lower limit of the weight-average molecular weight of the second crystalline polymer is 500, preferably 800, more preferably 1000, particularly 2000. The upper limit thereof is 10,000, preferably 9,000, more preferably 8,000.
Preferably, the lower limit of the melting point (Tm2) of the second crystalline polymer is 60xc2x0 C., more preferably 90xc2x0 C., most preferably 100xc2x0 C., e.g. 105xc2x0 C., particularly 110xc2x0 C., more particularly 115xc2x0 C., especially 120xc2x0 C., more especially 125xc2x0 C. Preferably, the upper limit of the melting point (Tm2) of the second crystalline polymer is 200xc2x0 C., more preferably 180xc2x0 C., especially 140xc2x0 C.
The crystallinity of the second crystalline polymer may be at least 20%, preferably at least 50%. The lower limit of the crystallinity of the second crystalline polymer may be 60%, particularly 70%, especially 80%. The upper limit thereof is not limited, and may be 98%, particularly 95%, especially 92%.
The second crystalline polymer has at least one repeat unit derived from a monomer having a carbon-carbon double bond. The second crystalline polymer can be synthesized by polymerizing at least one monomer selected from olefins or olefin derivatives. Preferably, the second crystalline polymer is a homopolymer or copolymer of olefin such as ethylene or propylene (e.g. polyethylene, polypropylene, ethylene/ethyl acrylate copolymer).
The upper limit of the amount of the second polymer is 50% by volume, e.g. 40% by volume, particularly 30% by volume, especially 20% by volume, based on the polymer mixture. The lower limit of the amount of the second polymer may be 2% by volume, particularly 5% by volume, especially 10% by volume.
The crystallinity of the polymer mixture may be at least 20%, generally at least 40%, e.g. at least 60%, particularly at least 70%, especially at least 80%.
Preferably, a difference of the difference in melting point between the first and second crystalline polymers is at most 50xc2x0 C., more preferably at most 30xc2x0 C., particularly at most 20xc2x0 C.
The weight-average molecular weight of the polymers (i.e. the first and second crystalline polymers) is measured by gel permeation chromatography (GPC) (in terms of polystyrene).
The crystallinity of the polymers (i.e. the first and second crystalline polymers, and the polymer mixture) is usually measured by DSC (differential scanning calorimetry). The crystallinity can be measured by another method, e.g. X-ray diffraction, if the crystallinity cannot be measured by DSC, for example, if the numeral value of the crystallinity is low.
The melting point of the polymers means a melting peak temperature as measured by DSC.
The electrically conductive polymer composition comprises a particulate electrically conductive filler. The particulate electrically conductive filler includes carbon black, graphite, other carbonaceous materials, metal, metal oxide, electrically conductive ceramic, electrically conductive polymer, and a combination thereof Examples of carbonaceous material are carbon black, graphite, glassy carbon and carbon beads. Examples of metal are gold, silver, copper, nickel, aluminum and alloys thereof Examples of metal oxide are ITO (indium-tin oxide), lithiummanganese complex oxide, vanadium pentoxide, tin oxide and potassium titanate. Examples of electrically conductive ceramic are carbide (for example, tungsten carbide, titanium carbide and complexes thereof), titanium borate and titanium nitride. Examples of electrically conductive polymer are polyacetylene, polypyrene, polyaniline, polyphenylene and polyacene.
Preferably, the amount of the particulate conductive filler is from 10 to 80% by volume, more preferably from 30 to 80% by volume, particularly from 30 to 70% by volume, based on the total volume of the electrically conductive polymer composition.
The electrically conductive polymer composition may comprise additional components, such as antioxidants, inert fillers, nonconductive fillers, crosslinking agents, such as radiation crosslinking agents (often referred to as prorads or crosslinking enhancers, e.g. triallyl isocyanurate), stabilizers, dispersing agents, coupling agents, acid scavengers (e.g. CaCO3), flame retardants, arc suppressants, coloring agents or other polymers. These components comprise generally at most 20% by volume, e.g. at most 10% by volume of the total volume of the composition.
Preferably, a ratio (xcfx81m/xcfx8120)of a volume resistivity (xcfx81m)at a melting point of the electrically conductive polymer composition (i.e. at a melting point (Tm1) of the first crystalline polymer) to a volume resistivity (xcfx8120) at 20xc2x0 C. of the electrically conductive polymer composition is at least 50, e.g. at least 100, particularly at least 300, especially at least 1,000.
A volume resistivity (xcfx8120, a volume resistivity at 20xc2x0 C.)of the electrically conductive polymer composition is generally at most 100 ohm-cm, e.g. at most 10 ohm-cm, particularly at most 1 ohm-cm, more particularly at most 0.25 ohm-cm, more especially at most 0.15 ohm-cm. The volume resistivity (xcfx8120) of the composition depends on the application and what type of electrical device is required. When, as is preferred, the composition is used for circuit protection devices, the composition has a lower resistivity.
The electrically conductive polymer composition and the PTC device of the present invention can be prepared as follows:
The first crystalline polymer, the second crystalline polymer and the particulate electrically conductive filler are charged into a mixing apparatus and kneaded at high temperature to give a molten mixture (that is, the electrically conductive polymer composition). The kneading temperature is a temperature higher than the melting points of the first and second crystalline polymers, and is generally from 120 to 250xc2x0 C. The mixing apparatus may be an extruder, such as a single screw extruder or a twin screw extruder, or other types of mixing equipment, such as Banbury(trademark) mixers and Brabender(trademark) mixers.
Then the molten mixture is shaped into a polymeric sheet. This can be achieved easily by extrusion through a sheet die or by calendering the molten mixture, i.e. passing the molten mixture between rollers or plates to thin it into a sheet. The thickness of the calendered sheet is determined by the distance between the plates or rollers, as well as the rate at which the rollers are rotating. Generally the polymeric sheet has a thickness of 0.025 to 3.8 mm, preferably 0.051 to 2.5 mm. The polymeric sheet may have any width. The width is determined by the shape of the die or the volume of material and rate of calendering, and is often 0.10 to 0.45 m, e.g. 0.15 to 0.31 m.
A laminate is formed by attaching metal foil to at least one side, preferably to both sides, of the polymeric sheet. When the laminate is cut into an electrical device, the metal foil layer(s) act(s) as an electrode. The metal foil generally has a thickness of at most 0.13 mm, preferably at most 0.076 mm, particularly at most 0.051 mm, e.g. 0.025 mm. The width of the metal foil is generally approximately the same as that of the polymeric sheet, but for some applications, it may be desirable to apply the metal foil in the form of two or more narrow ribbons, each having a width much less than that of the polymeric sheet. Suitable metal foils include nickel, copper, brass, aluminum, molybdenum, and alloys, or foils which comprise two or more of these materials in the same or different layers. Metal foils may have at least one surface that is electrodeposited, preferably electrodeposited nickel or copper. For some applications, an adhesive composition (i.e. a tie layer) may be applied to the polymeric sheet, e.g. by spraying or brushing, before contact with the metal foil. The laminate may be wound onto a reel or sliced into discrete pieces for further processing or storage. The thickness of the laminate is generally 0.076 to 4.1 mm.
When the laminate comprises two metal foils, it can be used to form an electrical device, particularly a circuit protection device. The device may be cut from the laminate. In this application, the term xe2x80x9ccutxe2x80x9d is used to include any method of isolating or separating the device from the laminate.
Additional metal leads, e.g. in the form of wires or straps, can be attached to the foil electrodes to allow electrical connection to a circuit. In addition, elements to control the thermal output of the device, e.g. one or more conductive terminals, can be used. These terminals can be in the form of metal plates, e.g. steel, copper, or brass, or fins, that are attached either directly or by means of an intermediate layer such as solder or a conductive adhesive, to the electrodes. For some applications, it is preferred to attach the devices directly to a circuit board.
In order to improve the electrical stability of the device, it is often desirable to subject the device to various processing techniques, e.g. crosslinking and/or heat-treatment. Crosslinking can be accomplished by chemical means or by irradiation, e.g. using an electron beam or a Co60 irradiation source. The level of crosslinking depends on the required application for the composition, but is generally less than the equivalent of 200 Mrads, and is preferably substantially less, i.e. from 1 to 20 Mrads, preferably from 1 to 15 Mrads, particularly from 2 to 10 Mrads for low voltage (i.e. less than 60 volts) circuit protection applications. Generally devices are crosslinked to the equivalent of at least 2 Mrads.
Devices of the invention are preferably circuit protection devices that generally have a resistance at 20xc2x0 C. of less than 10 ohms, preferably less than 5 ohms, particularly less than 2 ohms, more particularly less than 1 ohm, especially less than 0.5 ohms, more especially less than 0.1 ohm, most especially less than 0.05 ohm. Because the laminate prepared by the method of the invention comprises a conductive polymer composition which can have a low resistivity, it can be used to produce devices with very low resistances, e.g. 0.001 to 0.100 ohm.
The electrically conductive polymer composition of the present invention can be used as an overcurrent protection device (a circuit protection device), a PTC thermistor, a temperature sensor and the like.
The electrically conductive polymer composition of the present invention has a low melt viscosity and exhibits good PTC anomaly, even if a large amount of the particulate electrically conductive filler is loaded to give a decreased volume resistivity at normal temperature (for example, 20xc2x0 C.) of the composition. The electrically conductive polymer composition of the present invention has good processability, the thickness of the PTC device can be smaller and the speed of lamination of the electrically conductive polymer composition layer and electrode layers can be higher. In addition, the PTC device has good adhesion between the electrically conductive polymer composition layer and the electrode layers. The present invention gives a PTC device having a small size, a light weight and a low electrical resistance.
The devices of the invention are often used in an electrical circuit which comprises a source of electrical power (e.g. DC power source or AC power source), a load, e.g. one or more resistors, and the device. In order to connect the device of the invention to the other components in the circuit, it may be necessary to attach one or more additional metal leads, e.g. in the form of wires or straps, to the metal foil electrodes. In addition, elements to control the thermal output of the device, i.e. one or more conductive terminals, can be used. These terminals can be in the form of metal plates, e.g. steel, copper, or brass, or fins, which are attached either directly or by means of an intermediate layer such as solder or a conductive adhesive, to the electrodes.